The present invention relates to a method for reducing the consumption of a stepping motor by automatically adapting the duration of each driving pulse supplied thereto to the load which is to be driven by the motor in response to said driving pulse.
The present invention also relates to a device for carrying out the method.
Stepping motors are used in many devices in which a mechanical member is to be moved by a given amount in response to an electrical signal. They are used in particular in electronic timepieces in which the time display hands must be moved by a given distance in response to pulses of a highly precise period, which are supplied by a time base.
In such timepieces, the major part of the power supplied by the electrical power source, which is generally a battery, is consumed by the stepping motor. As the space available in timepieces is greatly restricted, it is important for the level of consumption of the motor to be limited as far as possible, in order to increase the service life of the battery or, for a given service life, to be able to reduce the space occupied thereby.
In most of the present-day timepieces, the duration of the driving pulses which are supplied to the motor at regular intervals is constant. The duration of the driving pulses is so selected as to ensure proper operation of the motor even under the worst conditions, that is to say, with a low battery voltage, while driving the calendar mechanism, when subjected to shocks or in the presence of an external magnetic field, etc. As such poor conditions occur only rarely, the motor is over-powered in most cases.
It is possible to substantially reduce the power consumption of the motor by adapting the power supplied by the driving pulses to the instantaneous load to be driven by the motor and to the supply voltage.
One solution to this problem comprises providing a pulse shaping circuit capable of producing pulses of different durations and a device for detecting rotation or non-rotation of the motor. The duration of the driving pulses applied to the motor is progressively reduced until the device detects that a step has not been performed. A catch-up pulse is then applied to the motor and the energy of the normal driving pulses is fixed at a higher value which is maintained for a certain period of time. If the motor has rotated normally during that period, the duration of the pulses is again reduced. Such a design does not provide for the driving pulses to be permanently and rapidly adapted to the load on the motor. In addition, this slow adaptation procedure and the production of catch-up pulses when the motor does not perform a stepping movement mean that the power consumption is higher than necessary.
In order to overcome this disadvantage, it is known to provide devices which adapt the duration of each driving pulse to the load to be entrained by the motor in response to the driving pulse.
U.S. Pat. No. 3,500,103 describes a device for detecting the movement of the movable member of the motor by way of the voltage induced in a detection coil which is separate from the drive coil, and which interrupts the driving pulse when the movable member reaches either a given position or a given speed.
U.S. Pat.No. 3,855,781 proposes solutions according to which the position of the rotor is detected by measuring the voltage induced in an auxiliary coil or caused by the deformation of a piezoelectric element under the action of the teeth of one of the wheels of the wheel-train which is driven by the motor. That voltage is used to interrupt the driving pulse.
The device described in the two patents referred to above require additional elements for operation thereof, which makes them expensive and complicated to use.
French Pat. No. 2 200 675 proposes detecting the variation in current in the actuating coil of the motor and interrupting the driving pulse when the current passes through a minimum. The limits of this detection operation are imposed by the form of the current which depends on the time constant of the circuit, the counter-electromotive force induced, and the load on the motor. In some cases, the current minimum may disappear, which renders the control device inoperative.
In addition, U.S. Pat. No. 4,114,364 describes a circuit for controlling the duration of the driving pulses in dependence on the load on the motor, which comprises means for detecting the current in the actuating coil and means for interrupting the pulse when that current reaches a value equal to the ratio between the supply voltage of the coil and its d.c. resistance, that is to say, when the rotor has concluded its stepping motion. Also provided is the possibility of interrupting the pulse before the current has reached that value.
All the above-described devices use measurement of a physical parameter such as the speed or position of the rotor or such as the current flowing in the coil. The measurement made is used directly or by comparison with a reference value, to control interrupting the driving pulse. Now, none of the above-mentioned physical parameters gives an absolute indication as to the precise moment at which the driving pulse is to be interrupted in order for the power consumption of the motor really to be at a minimum. All these devices therefore cause the driving pulse to be interrupted at an arbitrarily selected moment which is generally not the optimum moment. In practice, these devices must take account of safety factors such that, most of the time, the motor consumes too much energy or does not operate safely.